1. Field of the Invention
The present invention relates to a projector having an auto focus device.
2. Description Related to the Prior Art
Many kinds of projectors for projecting images to a screen such as a slide projector and a liquid-crystal projector are utilized. In back projection type projectors which incorporates the screen, there is no need to perform focusing of a projection lens for every use because a distance between the projection lens and the screen stays constant. However, in general projectors which projects images from front side of the screen, there is need to perform focusing of a projection lens according to the distance between the projection lens and the screen.
The projectors which incorporate an auto focus device to enable the automatic focusing of the projection lens are disclosed in Laid-Open Japanese Patent Applications 5-346569 and 2003-161869. As the auto focus device of the projector, it is often used that an active type distance measurement device which measures the distance between the projection lens and the screen by triangulation such that the device projects a distance measurement light to the screen which is the projection surface for the image and receives the diffuse reflected light from the screen.
As shown in FIG. 6, in general the projector 2 is mounted on a table 3, and the screen 4 is often set at high place so that the projected image can be seen easily. In this case, if the whole projector 2 is inclined upward for adjusting the center of the projected image and center of the screen 4, the projected image becomes a trapezoid shape because the image is expanded in the upper part of the screen. To avoid this problem, in the figure, the projection lens 5 is shifted upward with respect to center of a liquid crystal panel 6 which displays images for projecting with that the optical axis 5a of the projection lens 5 is horizontal (perpendicular to the screen 4). Accordingly, the strain-free image can be projected on the screen 4, in spite of that an optical axis 5b of center of the image, which connects the center of the liquid crystal panel 6 and the center of the screen 4, is inclined upward with respect to an optical axis 5a of the projection lens 5.
In the active type distance measurement device used in Laid-Open Japanese Patent Applications 5-346569 and 2003-161869, as described in FIG. 7, near-infrared light from an infrared emitting diode (IRED) 10 is projected to the projection surface 9 through a light emitting lens 11. In case that the screen is used as the projection surface 9, an spot image (spot area S1) is formed on the surface of the screen by irradiation of the distance measurement light, because the surface is fine coarse and has diffusion reflection property. A light receiving lens 12 is provided at a position distant from center of the light emitting lens by a base length L. The diffuse reflected light from the spot area S1 enters to the light receiving lens 12, and an spot image is focused on a photoelectric surface of a light receiving element 13 provided back of the light receiving lens 12. Note that it only needs that the spot image on the screen is in a field angle of the light receiving lens 12. Therefore, there is no need that an optical axis 12a of the light receiving lens 12 is parallel to an optical axis 11a of the light emitting lens 11, even though there often be a case that an optical axis 12a of the light receiving lens 12 is parallel to an optical axis 11a of the light emitting lens 11, as described in the figure.
In general, a PSD (Position Sensitive Detector) is used as the light receiving element 13, which outputs a pair of electrical signals corresponding to a position (center of gravity) of the spot image imaged by the light receiving lens 12. The PSD has a function to discriminate the position of incidence of light in direction of the base length L, and the image forming position of the spot image on the photoelectric surface corresponds to the distance between the projection lens and the screen by triangulation. Therefore, a distance measurement signal corresponding to the distance between the projection lens and the screen can be generated based on the pair of electrical signals from the PSD, with being immune to amount of light entered into the PSD. The automatic focusing is operated such that a focus motor is driven according to the distance measurement signal to move a focus lens of the projection lens system in direction of the optical axis 5a. 
Recently, whiteboards are often used as a substitute for the screen when the projector is used in offices. The whiteboard has a surface which is smoother and has higher reflectivity than the normal screen having diffusion reflection property, to enable writing down by markers and erasing by erasers. Accordingly, the distance measurement light emitted from the distance measurement device is reflected at a considerably high intensity at the whiteboard and enters into the light receiving element 13. In this time, the distance measurement light from the light emitting lens 11 to the projection surface 9 includes not only the effective distance measurement light formed as a beam. Because the light emitting lens 11 is not absolutely transparent to near-infrared light, the light emitting lens 11 emits diffused light with the effective distance measurement light. In addition, diffuse reflected light is often emitted from for example components mounted back of the light emitting lens 11.
As shown in FIG. 7, most of these noise lights are emitted with gradation of intensity as high as closer to a center of a surrounding area S2 which surrounds the spot area S1 to which the effective distance measurement light is emitted, and as low as closer to a periphery of the surrounding area S2. Ordinarily, the intensity of the noise light is very lower than that of the effective distance measurement light, therefore the noise light from the surrounding area S2 hardly reaches to the light receiving lens 12 when the normal screen is used as the projection surface 9. However, when the whiteboard is used, part of the noise light which is emitted to the surrounding area S2 becomes to have many components which are regularly reflected at the surface of the whiteboard (the light which has an exit angle being equal to an incidence angle). As shown in figure as “regularly reflected noise light”, part of diffused light reaches to the light receiving element 13 by entering into the light receiving lens 12 at an angle different from that of the effective distance measurement light with being little attenuated. As described above, the PSD which is often used as the light receiving element 13, outputs electrical signals corresponds to the incidence position of the distance measurement light with being immune to intensity of the distance measurement light, therefore the noise light becomes a major factor of erroneous distance measurement.
Even if the case that other photoelectric sensors are used as the light receiving element 13, it is also difficult to distinguish the distance measurement light and the noise light according to the intensity of the distance measurement light, because the normal screen or the whiteboard is used as the projection surface 9 according to situations, and the distance from the projector is different in different cases. In addition, measures for erroneous distance measurement from the noise light rises cost of the projector.
As an example of the distance measurement is executed with the condition that the distance between the projector and the whiteboard is 1 meter, and the light emitting optical axis 11a and the optical axis 12a of the light receiving lens 12 are parallel to the optical axis 5a of the projection lens 5, and with inclining the projector upward and downward. In FIG. 8, a horizontal axis represents projective angle of the projection lens to the whiteboard, in which the condition that the optical axis 5a of the projection lens 5 and the optical axis 11a of the light emitting lens 11 is perpendicular to the whiteboard is shown at [0°], the condition that the optical axis 11a is inclined mostly upward with respect to the whiteboard is shown at [−10°], and the condition that the optical axis 11a is inclined mostly downward with respect to the whiteboard is shown at [10°]. A vertical axis represents digital value of distance signal (larger value corresponds to shorter distance) calculated based on the signal from the light receiving element (PSD) 13, each value corresponds to the distance to the whiteboard one to one.
Because the distance between the whiteboard and the projector is constant at 1 meter, the distance signal is expected to be constant value (≈4875) even if the projection angle is changed. However, when the projection angle is between [−2°] and [3°], the distance signal abnormally depends on the projection angle, even though the mostly constant distance signal is obtained when the projection angle is more than 3° in upward or downward direction. The fact means that when the light emitting optical axis 11a is inclined at [−2° to 3°] with respect to the whiteboard, the possibility of occurring the erroneous distance measurement becomes high.